The present invention relates to a control method of a spark-ignition engine and a spark-ignition engine system, which comprise a spark plug to supply a spark to ignite an air-fuel mixture within a combustion chamber and a catalyst for purifying exhaust gas which is provided in an exhaust passage.
Conventionally, a catalyst-temperature control device, which comprises a catalyst provided in an exhaust passage of an internal combustion engine where combustion is performed under a lean air-fuel ratio, an exhaust control valve to control the amount of exhaust gas flowing into the catalyst, and another combustion device which is provided in addition to the internal combustion engine, is known as disclosed in Japanese Patent Laid-Open Publication No. 2003-120269. In this control device, when increasing the catalyst temperature is required, the opening of the exhaust control valve is controlled so that the amount of exhaust gas flowing into the catalyst from the engine can decrease, and the combustion device operates in such a manner that combustion therein is performed under a rich air-fuel ratio so that high-temperature exhaust gas exhausted from this device can be supplied into the catalyst.
The catalyst-temperature control device disclosed in the above-described publication can improve the thermal efficiency of the engine by the combustion under the lean air-fuel ratio. Further, since the high-temperature exhaust gas exhausted from the combustion device is supplied into the catalyst, a stable increase of the catalyst temperature can be provided even if the temperature of the exhaust gas from the engine is relatively low because of the combustion under the lean air-fuel ratio in the engine.
The device disclosed in the above-described publication, however, has a problem in that since the above-described combustion device is required additionally to the engine, the device may become complex as a whole and the number of parts or manufacturing costs may improperly increase.